So this old tube TV was beyond repair.
Again I decided to put a modern flat TV in it. The original speaker is still in use and I used a modern transistor amp that I put between the speaker and the TV.
The reason is that you can now still set the volume by turning the left knob^^
In the lower middle between the knobs I put a status LED which is a sign for Stand-by mode. And by pressing the original On/Off Key on the keyboard you can switch it on or off completely^^